Mold having vent passageways to obviate trimming of flash

ABSTRACT

A mold for producing foamed articles includes an upper mold and a lower mold which define a mold cavity. A vent is provided in communication with the mold cavity and has a vent passageway with an entrance and an exit. Structure is also provided for sealingly engaging the upper mold with the lower mold to cause, in cooperation with the predetermined size and shape of the vent passageway, liquid foam polymeric composition expanding inside the mold cavity to enter the vent entrance but to not exit from the vent exit to cause at least partial curing of the foam composition within the vent passageway. Preferably, the vent may be ribbon-shaped so that a ribbon of foamed composition adheres to the foamed article and may be folded flat against the article thus avoiding the labor step of trimming the flash from the product. Additionally, .[.or alternatively,.]. the vent may include a vent passageway with an obstruction disposed therein to provide one or more openings between the outer surface of the obstruction and the inner surface of the vent passageway. The foam composition in these openings may be expelled by driving the obstruction axially within the passageway. Again, the labor step of hand trimming the flash is avoided.

This application is a division of application Ser. No. 07/939,704 filedSep. 2, 1992, now U.S. Pat. No. 5,356,580.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel mold for producing foamedarticles. Specifically, an aspect of the present invention relates to animproved mold including a vent capable of providing necessary venting ofthe mold while minimizing extrusion of raw material into the vent, thusreducing wastage. In its most preferred embodiment, the presentinvention also eliminates the requirement to remove the extruded rawmaterial from the molded article prior to application of a finish (e.g.trim) cover.

The present invention also relates to a novel method of molding articleswhich reduces wastage resulting from raw material being extruded intothe vent during venting of the mold. In its most preferred embodiment,the method reduces or eliminates the requirement to remove the extrudedmaterial prior to application of a finish cover.

2. Description of the Prior Art

Many articles are manufactured by placing a raw material into a cavityin a mold wherein the raw material undergoes a physical change (e.g. itexpands or foams) and the article produced thus acquires the shape ofthe cavity. In particular, this technique is commonly employed forproducing foamed articles made from polymeric foams such as polyurethanefoam, latex (e.g. natural and styrene-butadiene rubber) foam and thelike.

For example, automotive seats are commonly manufactured frompolyurethane cushions which are molded to shape and then covered with avinyl, cloth or leather finish cover. Polyurethane foams are somewhatunique in that foaming and at least a portion of the polymerizationprocess occur simultaneously. Thus, in the production of polyurethanefoam using, for example, a conventional cold foam technique, a typicalformulation comprises:

1. Polyol

2. Water

3. Tetramethyl ethane diamine

4. Dimethyl ethanol amine

5. Polyisocyanate

The mixture is dispensed into a mold using a suitable mixing head, afterwhich the mold is then closed to permit the expanding mass within it tobe molded. Accordingly, it is convenient generally to refer to themixture initially dispensed into the mold as "a liquid foamablepolymeric composition" or, in this case, "a liquid foamable polyurethanecomposition". As the composition expands in the mold, polymerizationoccurs and the polymer so formed becomes solidified.

When molding a liquid foamable polymeric composition to form articlessuch as polyurethane foam articles, it is conventional to use aclam-shell mold comprising a bottom mold and a top mold which, whenclosed, define a mold cavity. The mold is opened, the liquid foamablepolyurethane composition is dispensed into the mold cavity and the moldis closed as a chemical reaction causes the composition to expand. Afterthe mold is closed, the composition expands to fill the interior cavityof the mold. Alternatively, the composition may be dispersed into aclosed mold. In either case, as the polymerization reaction iscompleted, the foam cures and permanently assumes the shape of the moldcavity.

As is known to those of skill in the art, it is essential during thisprocess that the mold be adequately vented to allow the air present inthe mold to exit the mold as the foamable composition expands. Further,it is essential to allow a portion of the gases (typically CO₂ in theproduction of polyurethane) generated during polymerization to exit themold.

Failure to adequately vent the mold results in defective molded articlesexhibiting symptoms of improper foaming such as surface hardening (orfoam densification) and/or void formation in the finished article due totrapped gas or air bubbles. At the other extreme, excess venting of themold will also result in defective molded articles due to collapse ofthe foam prior to curing; this phenomenon is often referred to as the`souffle` effect. Thus, proper venting of molds is an important factorin producing acceptable molded articles.

Typically clam-shell molds are designed with drilled or cut passages inthe top mold to provide vents. Locating, sizing and deciding upon thenumber of these vents is a matter of some skill on the part of molddesigner and the production engineers, and is often an iterativeprocedure with more vents being added to various locations or othervents being blocked-off after test runs have been made.

During molding operations some liquid foamable polymeric compositionwhich moves into the vent is wasted. It is generally desired to minimizethe amount of wasted material (also known as "flash", "mushrooms","buds", "pancakes" and the like) for two masons, namely (1) the wastedmaterial adds to the overall expense of producing the finished article,and (2) the wasted material must be removed from the molded articleprior to the finish cover being applied, thereby necessitatingadditional labour and the costs associated therewith.

Accordingly, mold designers and production engineers are continuallystriving to optimize the compromise between providing enough venting atthe proper locations while avoiding excess venting and minimizingmaterial wastage during venting.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel method ofproducing a molded article which obviates or mitigates material wastageduring venting.

It is another object of the present invention to provide a novel moldwhich obviates or mitigates at least some of the above-mentionedproblems with prior art.

It is yet another object of the present invention to provide a novelmethod of producing a molded article from which vent-extruded materialneed not be removed.

According to a first aspect of the present invention, there is provideda method of producing an article in a mold comprising an upper mold anda lower mold defining a mold cavity, the process comprising the stepsof:

dispensing a liquid foamable polymeric composition into the mold cavity;

allowing the liquid foamable polymeric composition to expand tosubstantially fill the mold cavity; and

venting gases in the mold cavity through at least one vent in the moldto allow the gases to exit from the mold, the size of each vent beingselected such that movement of the liquid foamable polymeric compositioninto the vent is restricted to substantially prevent exit thereof fromthe vent.

Preferably, the at least one vent is located at the part-line of themold and comprises a thickness in the range from about 0.002 inches to0.030 inches, more preferably in the range from about 0.005 inches toabout 0.020 inches. Also preferably, the at least one vent isrectangular in cross-section and the polymeric material which enters thevent forms a ribbon of material on said article which need not beremoved when a finish cover is applied to said article.

According to a second aspect of the present invention, there is provideda mold for producing foamed articles, the mold comprising:

an upper mold and a lower mold which define a mold cavity; and

at least one vent in communication with the mold cavity, the size ofeach vent being selected to allow relatively free flow of gasestherethrough and to restrict movement of a liquid foam polymericcomposition therethrough.

Preferably, the vent includes first and second vents, the first ventbeing located at the part-line between the top and bottom molds. In thisembodiment, it is preferred that the first vent comprises a thickness offrom about 0.002 inches to about 0.030 inches, more preferably fromabout 0.005 inches to about 0.020 inches. It is preferred that thesecond vent is located in the upper mold and comprises a thickness offrom about 0.002 inches to about 0.015 inches, more preferably fromabout 0.003 inches to about 0.010 inches.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described with reference tothe accompanying drawings, in which:

FIG. 1 illustrates a cross-section of a prior an clam-shell mold;

FIG. 2 illustrates a sectional view of an article produced in the priorart mold of FIG. 1;

FIG. 3 illustrates a cross-section of a vent assembly;

FIG. 4 illustrates a partial section taken along line 4--4 in FIG. 3;

FIG. 5 illustrates the vent assembly of FIG. 3 in a cleaning position;

FIG. 6 illustrates a cross-section of a clam-shell mold employing thevent assemblies shown in FIG. 3;

FIG. 7 illustrates a perspective view of another clam-shell mold;

FIG. 8 illustrates a section taken along line 8--8 when the mold of FIG.7 is closed;

FIG. 9 illustrates a molded article made in the mold shown in FIG. 7;and

FIG. 10 illustrates a cross-section of a mold employing the ventassemblies shown in FIG. 3 and the vents shown in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The most preferred liquid foamable polymeric composition is based uponpolyurethane. However, it will be apparent to those of skill in the artthat the present invention is applicable to other types of moldingoperations including, but not limited to, latex foam, neoprene foam, PVCfoams and the like.

A prior art mold will first be discussed, with reference to FIGS. 1 and2. A typical clam-shell mold, similar to those used for forming anautomotive seat cushion from polyurethane foam, is indicated generallyat 20 in FIG. 1. The mold includes a lower mold 24 and an upper mold 28which are joined by a conventional hinge means (not shown). Lower mold24 and upper mold 28, when closed, define a cavity 32 which correspondsto the shape of the automotive seat cushion.

In use, upper mold 28 is released from lower mold 24 and a predeterminedamount of liquid foamable polyurethane composition is dispensed intolower mold 24. Upper mold 28 and lower mold 24 are then sealinglyengaged and the liquid foamable polyurethane composition expands,displacing the air within cavity 32. This displaced air exits cavity 32through a part-line 36 and through one or more vent passages 38 in uppermold 28. Further, as the polyurethane composition expands,polymerization occurs along with the evolution of gaseous CO₂ in cavity32. This gaseous CO₂ may also exit cavity 32 through part-line 36 andthrough vent passages 38. As is well known to those of skill in the art(and beyond the scope of this discussion), the liquid foamable polymericcomposition eventually completely polymerizes and cures, acquiring theshape of cavity 32.

As is also known to those of skill in the art, the amount of liquidfoamable polyurethane composition dispensed in cavity 32 must beselected to ensure that cavity will be substantially completely filled,in order to avoid the occurrence of voids and other foaming defects inthe molded article. While the determination of the proper amount ofliquid foamable polyurethane composition for a particular mold maygenerally be calculated, as explained below it has heretofore beenrequired to dispense an excess amount of polymeric composition into themold to compensate for material which moves through and exits part-line36 and vent passages 38. This excess, while assisting in ensuring thatcavity 32 is filled to avoid the occurrence of voids and other foamingdefects in the molded articles, is in fact simply a wastage of valuableraw material.

In prior .[.an.]. .Iadd.art .Iaddend.molds, during the moldingoperation, air and the reaction gases produced from the expandingcomposition exit from cavity 32 through part-line 36 and vent passages38 until the foam reaches the level of their respective entrances.

At this point, any further expansion of the foam results in movement ofthe foam into the part-line 36 and/or vent passages 38. In the simplestcase of a cavity without irregularities, the foam reaches the level ofthe part-line and/or the vent passages at approximately the same time,which usually occurs at or near the maximum expansion point of the foam.Thus, provided that the proper amount of liquid foamable polyurethanecomposition has been dispensed into the cavity, only a small amount offoam enters the part-line and/or the vent passages as cavity 32 iscompletely filled.

In practice, however, as shown in FIG. 1, most molds includeirregularities in their cavities for various features required on themolded article. In such a case, the thickness and shape of mold cavity32 typically varies across the cavity and the entrance to part-line 36and vent passages 38 in the mold may thus be located at differentheights depending upon where they communicate with cavity 32. Further,localized areas of high and low pressure also occur within cavity 32 dueto the manner in which the foam and the gases produced collect in andmove between the irregularities therein and thus the level of foam indifferent parts of cavity 32 at different times may vary.

Due to the above-mentioned factors, the foam in the cavity typicallyreaches the level of the part-line and/or different vent passages atdifferent times while the foam is still expanding. For example, in aregion wherein the top of cavity 32 is lower than surrounding regions,such as indicated at 40 in FIG. 1, the foam may quickly reach the ventpassages 38. As the foam is still rising in the rest of cavity 32 andhas not yet cured, a relatively significant amount of foam may entervent passages 38 in this region.

Again, as the amount of foam which enters the part-line 36 and ventpassages 38 reduces the amount of foam remaining in cavity 32 by a likeamount, it is necessary that the amount of liquid foamable polyurethanecomposition placed in cavity 32 include an amount in excess of thatrequired to fill cavity 32 to offset the foam which entered thepart-line and vents. This excess amount, while necessary for properoperation of the prior art mold, is essentially wasted material and addsto the cost of forming the article.

Further, as shown in FIG. 2, the foam which enters vents 38 forms`mushrooms` 54 (shown in ghosted line) of wasted material on the moldedarticle 50. Further, the material which enters part-line 36 forms`pancakes` 55 of wasted material on the moulded article 50. Typically,mushrooms 54 and pancakes 55 must be disconnected from article 50 andremoved from the mold 20 prior to application of a finish cover toensure a finished covered article which is of acceptable appearance andtexture, and to prepare mold 20 for re-use. The necessity of removingmushrooms 54 and pancakes 55 results in an increased labour costassociated with manufacturing the molded product.

In addition to the excess liquid foamable polyurethane composition whichis added to offset the material extruded into the vents, excess liquidfoamable polyurethane composition is also added to compensate forprocess variations due to changes in temperature, humidity, ambientpressure and minor changes in the composition of the liquid foamablepolyurethane composition. Accordingly, in prior art molds, the wastageof material exiting the vents is inevitable.

Embodiments of the present invention will now be described, withreference to FIGS. 3 through 10.

The present inventors have determined that it is possible to use thedifference in the physical characteristics of the expanding liquidfoamable polymeric composition, prior to curing thereof, and the ventedgases to design vents which minimize the amount of material which isextruded into the vent passages as wasted material. Specifically, it hasbeen determined that vents may be designed with sizes which provide muchdifferent flow rates depending on the viscosity of the fluid flowingtherethrough. Thus, gases, which possess a relatively low viscosity,flow relatively easily and quickly through the vent. In contrast, theliquid foamable polymeric composition, which possesses a relatively highviscosity (especially when it reaches the vent as expansion and curingate near completion), flows much more slowly through the vent due to therestriction the thickness of the vent presents to the foam.

As described hereinafter, the size of the vents is selected such thatthe gases in the mold may flow through the vents in a relativelyunhindered manner while the viscosity of liquid foamable polymericcomposition, when it reaches the vent, is such that it flows into andthrough the vents in a very slow manner, at best. Provided that the ventsizes are properly selected, the liquid foamable polymeric compositionwill have cured before it has flowed a significant distance into thevent.

As will be apparent to those of skill in the art, vents in accordancewith the present invention thus reduce the amount of excess liquidfoamable polymeric composition which must be dispensed in the mold tooffset the material extruded into the vents.

FIGS. 3, 4, 5 and 6 illustrate a fast embodiment of an improved ventassembly 98 in accordance with the present invention which is preferredfor use in an upper mold. The vent assembly comprises a mold sleeve 100with a guide bushing 125 with a cylindrical bore 104 therethrough. Arelief pin 108 is located within and engages the inner surface of bore104. Relief pin 108 is octagonal in cross-section, as best seen in FIG.4, and when located within guide bushing 125 defines eight vent passages112 which are segment shaped (i.e., which are shaped as portions cutfrom a geometric figure (e.g. a circle) by one or more points, lines orplanes).

The upstream end 120 of relief pin 108 which communicates with thecavity of a mold is preferably tapered and the downstream end includes anarrowed throat 124. As shown in FIG. 3, when relief pin 108 is in theventing position, the downstream end of vent passages 112 are incommunication with vent chamber 127 which is in turn in communicationwith vent outlets 128. Vent outlets 128 exhaust gases to the environmentoutside the mold. In the operating position shown in FIG. 3, gases to bevented from a mold cavity travel along vent passages 112 to vent chamber127 and then exit through vent outlets 128.

The dimensions of vent passages 112 are selected to allow gases torelatively freely exit cavity 202 and to restrict movement of foamwithin vent passages 112. Specifically, due to the relatively highviscosity of the foam, the thickness 116 of vent passages 112 present arestriction to the foam while not restricting the gases. Provided thatthickness 116 is properly selected, the width 119 of vent passage 112 isnot particularly limited, nor is the shape of the passage 112.

As will be apparent to those of skill in the art, the diameter ofcylindrical bore 104 and cross-sectional shape of relief pin 108 may bevaried to provide different total vent areas while maintaining thedesired vent thickness. For example, relief pins with between four andeight sides, defining a like number of vent passages, have beensuccessfully tested.

For polyurethane foams, it has been found that a vent passage thicknessof less than about 0.010 inches present a suitable restriction. It willbe understood by those of skill in the art that the thickness of thevent passage(s) may be varied according to the particular liquidfoamable polymeric composition being used. Thus, if a polymer other thanpolyurethane is being foamed, the thickness may be suitably determinedas would be apparent to those of skill in the art through empiricalcalculations and/or testing.

While vent assembly 98 has been found to be very successful at limitingfoam extrusion from the mold, a small amount of foam does enter ventpassages 112. Thus, it is necessary to clean vent passages 112 prior tore-use of the mold. Accordingly, the downstream end of relief pin 108 isconnected to a piston 136 in a pneumatic cylinder 140. By varying thepressure on either side of piston 136 through apertures 148 or 152, theend of relief pin 108 may be extended or retracted within bore 104. Asrelief pin 108 is extended from vent bore 104, the downstream edge ofthroat 124 sweeps the interior of vent bore 104, removing residualextruded foam. As will be understood by those of skill in the art, guidebushing 125 may be fabricated from DELRIN (acetal resin) or any othersuitable material. FIG. 5 illustrates relief pin 108 in its extendedposition and FIG. 3 illustrates relief pin 108 in its operatingposition.

FIG. 6 illustrates a mold 200 which includes several vent assemblies 98.Preferably, vent assemblies 98 are installed in an upper mold 204 ofmold 200 in locations strategically selected to provide the desireddegree of venting. In use, gases are vented from the vent assemblies 98,as described above, until the foam within cavity .[.208.]. .Iadd.202.Iaddend.reaches the entrance to the vent. At this point, as the foambegins to enter vent passages 112, the restriction presented by ventpassages 112 on the foam slows the flow of foam into vent passages 112such that the foam will have cured before it has entered vent passage toa significant degree, and such that the foam does not exit the vent.

The above-mentioned embodiment provides an advantage over the prior artin that it reduces the amount of excess liquid foamable polymericcomposition required to accommodate the foam extruded into the moldvents. Accordingly, the amount of material which enters vent assemblies98 is reduced when compared to that obtained from conventional moldvents. This results in materials savings and may also results in laboursavings since this material need not always be removed from the articleor the mold.

Another embodiment of the present invention, will now be described withreference to FIGS. 7, 8 and 9 which is particularly preferred for use inthe part-line between the upper and lower molds. A clam-shell mold isindicated generally at 400 in FIG. 7. The mold includes a lower mold 404and an upper mold 408 which define a mold cavity 412. The mold includesa series of ribbon vents 416 in accordance with the present invention.As shown in FIG. 8, when mold 400 is closed, ribbon vents 416 extendbetween mold cavity 412 and the exterior of mold 400. It has been foundthat ribbon vents with a thickness indicated at 418, of about 0.005inches to about 0.015 inches are particularly suited for use whenmolding polyurethane foams. As with vent passages 112 of the previousembodiment, when used with other polymeric foamable compositions,thickness 418 may be altered as required.

The width, indicated at 417, of ribbon vents 416 is not particularlylimited. Widths of up to about 6.0 inches have been found to work in asatisfactory manner. In practice, it is contemplated that the limits tovent width 417 will primarily relate to physical limitations imposed bythe shape of the article being molded and the requirement to provide anadequate total venting area to cavity 412.

In use, liquid foamable polyurethane composition is dispensed into moldcavity 412, and upper mold 408 and lower mold 404 are sealingly engaged.The air in mold cavity 412 and the gases produced by the chemicalreaction occurring in the expanding composition are vented throughribbon vents 416. The viscosity of these gases are such that they flowrelatively easily through ribbon vents 416. Once the level of foam inmold 400 reaches the entrance to ribbon vents 416, the foam entersribbon vents 416. Due to the above-mentioned restriction, which ispresented to the expanding composition by vent 416, the expandingcomposition can only move slowly through ribbon vents 416. Provided thatthe thickness of ribbon vents 416 has been properly selected, the liquidfoamable polymeric composition will stop moving therein before ittravels a significant distance along the vents and certainly before itreaches the end of ribbon vents 416.

FIG. 9 illustrates an article 420 fabricated in the mold of FIG. 7employing ribbon vents 416 in accordance with the present invention. Asillustrated, article 420 includes a number of ribbons 424 of extrudedmaterial from ribbon vents 416.

The advantages provided by ribbon vents 416 are many. First, the amountof material extruded into ribbon vents 416 is limited due to therestriction presented by thickness 418 to the foam, thus the amount ofraw material wasted is reduced with a resultant economic saving. Second,ribbon vents 416 are relatively inexpensive to manufacture compared tothe prior art vents. Third, ribbon vents 416 may be added readily atregular spaced intervals about mold cavity 412, limited only by theshape of cavity 412, doing away with much of the iterative design efforton .[.be half.]. .Iadd.behalf .Iaddend.of the mold designer andproduction engineers which was previously required. Fourth, ribbon vents416 are easy to clean and in many circumstances are self-cleaning withribbons 424 being removed from vents 416 when the article 420 producedis removed. Fifth, ribbons 424 of extruded material produced by theribbon vents 424 have a preferred, `friendly` shape. Specifically, whenribbon vents 416 with relatively small thickness 418 are employed, theresulting ribbons 424 of extruded material may simply be folded backagainst article 420 when the finish cover is applied to it whilemaintaining an acceptable appearance and texture. An example of suchfolded back ribbons is indicated at 424a. This obviates the need toremove the ribbons 424 of extruded material and results in a laboursavings.

In some circumstances, it is contemplated that it may be desirable tosandblast or otherwise roughen the inner surfaces of ribbon vents 416 tofurther decrease the rate at which the liquid foamable polymericcomposition moves through the vent. Also, it is contemplated that insome circumstances ribbon vents with a thickness of less than 0.002inches may be employed and that such vents will inhibit substantiallyall foam extrusion into the vent. In this manner, the vent acts as adifferential vent allowing passage of gas but inhibiting passage offoam. However, care must be taken when using such differential vents toensure that an adequate total venting area is still provided to avoiddefects in the molded article.

It is contemplated that in many circumstances, it will be desired toemploy both of the above-described embodiments in a single mold.Specifically, as shown in FIG. 10, a mold 600 may include one or moreotherwise isolated regions 620 at which vent assemblies 98 maypreferably be employed, while ribbon vents 416 are preferably employedat the part-line of the mold. Due to their isolation from ribbon vents416 at the part-line of the mold, vent assemblies 98 are employed toensure that gases which would otherwise be trapped in regions 620 areproperly vented. The design of such a mold, will be clearly understoodby those of skill in the art, in view of the description above.

As will also be understood by those of skill in the art, furthervariations are possible without departing from the spirit of theinvention disclosed herein.

What is claimed is:
 1. A mold for producing foamed articles,comprising:an upper mold and a lower mold which define a mold cavity; atleast one vent disposed in said upper mold and in communication with themold cavity, said vent having a vent passageway with an entrance and anexit; an obstruction disposed in said vent passageway, said obstructionand said vent passageway combining to form at least one opening havingan elongated shape which allows movement of a liquid foam polymericcomposition to pass from the mold cavity into the vent passagewayentrance but prevents the liquid foam polymeric composition from exitingthe vent passageway exit to cause at least partial curing of the liquidfoam polymeric composition in the at least one opening between the ventpassageway and the obstruction; .[.and.]. driving means for driving theobstruction axially within said vent passageway to expel the at leastpartially cured liquid foam polymeric composition from said at least oneopening .Iadd.said lower mold having a first plurality of vents disposedalong one side of the lower mold at a part line between the upper andlower molds; said lower mold having a second plurality of vents disposedon a second side of the lower mold which is adjacent the first side,said second plurality of vents being disposed on the part line betweenthe upper and lower mold; and each of said first and second pluralitiesof vents having a vent passageway with an entrance and an exit, eachsaid vent passageway being sized and dimensioned to cause liquid foampolymeric composition expanding inside the mold cavity to enter intoeach said vent passageway entrance but to not exit from each said ventpassageway exit so that at least partial curing of the liquid foampolymeric composition occurs within each said vent passageway so thatthe at least partially cured liquid foam polymeric composition in eachsaid vent passageway does not separate from the mold foam article whenthe upper mold and lower mold are separated so as to produce theplurality of appendages on the sides of the foamed article. .Iaddend. 2.A mold according to claim 1, wherein said obstruction has an octagonalcross-section so that eight openings are disposed between saidobstruction and said vent passageway.
 3. A mold according to claim 1,wherein said driving means comprises a pneumatic piston.
 4. A moldaccording to claim 1, wherein said vent passageway comprises a plasticmaterial.
 5. A mold according to claim 4, wherein said plastic materialcomprises an acetal resin guide bushing.
 6. A mold according to claim 1,further comprising means, coupled to said obstruction, for sweeping aninterior of said vent passageway to expel liquid foam polymericcomposition adhering to the interior of said vent passageway when saiddriving means drives said obstruction.
 7. A mold according to claim 1,further comprising a plurality of corresponding vents, obstructions, anddriving means.
 8. A mold for producing foamed articles, comprising:anupper mold and a lower mold which define a mold cavity; at least onefirst vent disposed in said upper mold and in communication with themold cavity, said first vent having a vent passageway with an entranceand an exit; an obstruction disposed in said first vent passageway, saidobstruction and said first vent passageway combining to form at leastone opening having an elongated shape which allows movement of a liquidfoam polymeric composition expanding in the mold cavity to pass from themold cavity into the first vent passageway entrance but prevents theliquid foam polymeric composition from exiting the first vent passagewayexit to cause at least partial curing of the liquid foam polymericcomposition in the at least one opening between the first ventpassageway and the obstruction; driving means for driving theobstruction axially within said first vent passageway to expel the atleast partially cured liquid foamed polymeric composition from said atleast one opening; .Iadd.and .Iaddend. at least one second vent disposedon a part line between said upper mold and said lower mold and incommunication with the mold cavity, the second vent having a second ventpassageway with an entrance and an exit, the second vent passagewaybeing elongated with a width substantially greater than a thicknessorthogonal to the width to cause the liquid foam polymeric compositionexpanding inside the mold cavity to enter the second vent passagewayentrance but to not exit from the second vent passageway exit to causeat least partial curing of the liquid foam polymeric composition withinthe second vent passageway.
 9. A mold according to claim 8, wherein saidsecond vent passageway has a thickness orthogonal to the part line offrom about 0.002 inches to about 0.030 inches.
 10. A mold according toclaim 8, wherein the second vent passageway has a thickness orthogonalto the part line of from about 0.005 inches to about 0.020 inches.
 11. Amold according to claim 8, wherein said second vent passageway has asubstantially rectangular cross-section with a width parallel to thepart line of substantially 6.0 inches and a thickness orthogonal to thepart line of from about 0.005 inches to about 0.020 inches.
 12. A moldaccording to claim 8, wherein said driving means comprises a pneumaticpiston.
 13. A mold according to claim 8, wherein said first ventpassageway comprises a plastic material.
 14. A mold according to claim13, wherein said plastic material comprises an acetal resin guidebushing.
 15. A mold according to claim 8, further comprising means,coupled to said obstruction, for sweeping an interior of said first ventpassageway to expel liquid foam polymeric composition adhering to theinterior of the second vent passageway when said driving means drivessaid obstruction.
 16. A mold according to claim 8, further comprising aplurality of second vents disposed on the part line between said uppermold and said lower mold, and a plurality of first vents disposed insaid upper mold.